Example 2

var enter1, enter2 = false : boolean ;
    turn = 1 : integer ; {i.e. P1}

Process P1
 loop
 {entry protocol}
 enter1 := true; {announce intent to enter}
 turn := 2 ; {set priority to P2}

 while enter2 and turn=2
   do nothing ; {wait if other process in CS}

 {critical section}

 enter1 := false {exit protocol}
 end
end

Process P2
 loop
 {entry protocol}
 enter2 := true ; {announce intent to enter}
 turn := 1 ; {set priority to other process}

 while enter1 and turn=1
  do nothing ; {wait if other process is in CS}

 {critical section}

 enter2 := false ; {exit protocol}
 end
end

From Example 1 :

• Busy Wait
• Deadlock ?
  P1 sets enter1 to true
  P2 sets enter2 to true
  Both processes in busy wait loop
• Busy wait acceptable if anticipated waits are brief ? Costly !
• Alternation (entry into CS) between processes

From Example 2 :

• Break Deadlock - concurrent assignment to turn results in one of the processes having priority
• The favoured process concept - and alternate the favoured process (turn) variable

Aim to achieve synchronisation : simplest way -